The present invention relates generally to orthopedic implants. In particular, the present invention is discussed in connection with the tibial component of a unicondylar knee implant system, although the invention is not limited to just that type of component.
Orthopedic knee implant systems have been used for many years to treat patients with knee joints that have been damaged by trauma or disease, such as osteoarthritis, rheumatoid arthritis, and avascular neurosis. A knee arthroplasty procedure generally involves resecting, cutting, or resurfacing the damaged sections of the knee and replacing them with an endoprosthetic or implant.
Most knee implant systems are tricompartmental or total implants and the surgical procedure used with such implants is commonly known as total knee arthroplasty. These implants are known as tricompartmental implants because they are used when the knee joint is prepared to receive an implant by resurfacing or resecting the three articulating compartments, i.e., the medial and lateral femorotibial and the patellofemoral surfaces. Regardless of the type of implant used, arthroplasties generally require the bone to be specifically prepared to receive a corresponding implant by resecting, cutting, resurfacing, or otherwise deforming the bone to accept the implant.
Unicondylar or unicompartmental knee implants have become of great interest in the orthopedic industry due to their less invasive nature and the maintaining of the other healthy knee compartments. Unicondylar knees typically resurface or resect the medial or lateral femorotibial articulating surfaces thus allowing preservation of the other compartments not suffering from damage due to trauma or disease.
Historically, orthopedic devices have been mated with host bone by cementing them in place using methyl methacrylate, generally termed bone cement. The use of bone cement in attaching a prosthesis within or onto a prepared bone provides an excellent immediate fixation but has various disadvantages that appear over time. Physical loads are repeatedly applied to the implant over its life. If bone cement is used to secure a unicompartmental knee prosthesis, the bone cement may fatigue and fracture under the repeated loading. In some instances, degradation of the bone cement integrity may cause the device to become loose, thereby necessitating replacement. Old bone cement must be removed from the host bone as part of the implant replacement procedure. This procedure can be complex, time consuming and potentially destructive to healthy bone structures surrounding the implant. Furthermore, conventional bone cement is cured after it has been dispensed into the patient's joint. Loose undetected cement fragments can remain in the joint space and, with patient mobility over time, increase the degradation rate of articulating implant surfaces.
More recently, the development of orthopedic implant designs has moved towards satisfying the requirements of high demand patients. Patients today require more from their implants, and because patients are living longer, they require implants that to last longer. Accordingly, developments have been made in materials used to make orthopedic implants to improve implant longevity, such as highly porous metals that improve biological bone fixation. These implants are generally termed press-fit or cementless.
Recognizing the disadvantages of cement fixation techniques, prior art devices have also been developed that utilize other mechanical attachment means to join an implant to bone for immediate stabilization. Although various implant surface treatments intended to bond with bone biologically for long term stable attachment have proven successful, an initial fixation and stabilization is required before the bone growth can occur. A simple technique of mechanically securing an implant, is to affix it within the bone with screws or other mechanical fasteners. However, due to the nature of the bone surrounding the surgical site, and other limiting factors such as artery location and the like, screws can only be applied in certain limited regions. The use of a screw for implant fixation should be considered only as an option by the surgeon depending upon implant placement and bone quality.
Therefore, there exists a need for an improved implant design that provides both short term and long term fixation and stabilization.